As the geometries for integrated circuits have scaled to smaller and smaller dimensions, it has become necessary to replace polysilicon transistor gates with metal gates to enable scaling to continue to smaller dimensions. When voltage is applied to a polysilicon gate, the polysilicon grains next to the gate dielectric become depleted of carriers increasing the electrical thickness of the gate dielectric and exacerbating short channel effects. Metal gates do not deplete when voltage is applied to the metal gate.
Because the work function of most metal gates changes when the metal gate is subjected to high temperatures such as is required to activate dopants, replacement metal gate processes have been developed to circumvent the work function problem. In a replacement gate process, transistors are first built in the usual manner using polysilicon gates and silicon dioxide gate dielectric. The polysilion gates and gate dielectric are then removed and replaced with high-k gate dielectric and metal gates. Typically a thin silicon dioxide dielectric is grown on the single crystal silicon transistor channel prior to deposition of the high-k gate dielectric. Because silicide is typically on the wafer when the thin silicon dioxide is grown, the temperature at which this thin silicon dioxide may be grown is limited. The quality the thin silicon dioxide gate dielectric formed at low temperatures during the replacement gate process may be marginal.